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Date:   Tue, 01 Mar 2022 19:32:47 -0800
From:   Joao Moreira <joao@overdrivepizza.com>
To:     Peter Collingbourne <pcc@google.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Kees Cook <keescook@chromium.org>, x86@kernel.org,
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Subject: Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups
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On 2022-03-01 19:06, Peter Collingbourne wrote:
> Hi Peter,
> 
> One issue with this call sequence is that:
> 
> On Fri, Feb 11, 2022 at 02:38:03PM +0100, Peter Zijlstra wrote:
>> caller:
>> 	cmpl	$0xdeadbeef, -0x4(%rax)		# 7 bytes
> 
> Because this instruction ends in the constant 0xdeadbeef, it may
> be used as a "gadget" that would effectively allow branching to an
> arbitrary address in %rax if the attacker can arrange to set ZF=1.
> 
>> 	je	1f				# 2 bytes
>> 	ud2					# 2 bytes
>> 1:	call	__x86_indirect_thunk_rax	# 5 bytes
>> 
>> 
>> 	.align 16
>> 	.byte 0xef, 0xbe, 0xad, 0xde		# 4 bytes
>> func:
>> 	endbr					# 4 bytes
>> 	...
>> 	ret
> 
> I think we can avoid this problem with a slight tweak to your
> instruction sequence, at the cost of 2 bytes per function prologue.
> First, change the call sequence like so:
> 
>  	cmpl	$0xdeadbeef, -0x6(%rax)		# 6 bytes
> 	je	1f				# 2 bytes
> 	ud2					# 2 bytes
> 1:	call	__x86_indirect_thunk_rax	# 5 bytes
> 
> The key difference is that we've changed 0x4 to 0x6.
> 
> Then change the function prologue to this:
> 
> 	.align 16
> 	.byte 0xef, 0xbe, 0xad, 0xde		# 4 bytes
> 	.zero 2					# 2 bytes
> func:
> 
> The end result of the above is that the constant embedded in the cmpl
> instruction may only be used to reach the following ud2 instruction,
> which will "harmlessly" terminate execution in the same way as if
> the prologue signature did not match.

FWIIW, this makes sense IMHO. These additional pre-prologue bytes are 
also what might be needed to enable the smaller version of FineIBT that 
I suggested in this thread some time ago.